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We compare local electronic structure at different atom types of a metallic semiconductor in bulk and nanocrystalline form. Multinuclear magic-angle-spinning nuclear magnetic resonance (MAS NMR) establishes that GaN synthesized as an intentionally doped bulk powder or as annealed nanocrystalline particles exhibits metallic behavior and a wide distribution of differing electronic environments in both forms. Bulk polycrystalline wurtzite GaN doped with 0.13% Ge as a shallow donor exhibits a temperature-independent distribution of $^{71}\mathrm{Ga}$ Knight shifts over the temperature range 123--473 K. Each Knight shift frequency in the inhomogeneously broadened spectrum is characterized by a $^{71}\mathrm{Ga}$ spin-lattice relaxation time ${T}_{1}$ that is in good agreement with the value predicted by the Knight-Korringa relation across the broad range of temperatures. The $^{14}\mathrm{N}$ spectrum shows a slightly smaller Knight shift distribution with spin-lattice relaxation time ${T}_{1}$ values at 295 K across the distribution also in good agreement with the Knight-Korringa relation. Similarly, annealed nanocrystalline wurtzite GaN (50--100 nm, and without Ge) exhibits a $^{71}\mathrm{Ga}$ Knight shift distribution and ${T}_{1}$ values (at 295 K) that follow the same Knight-Korringa behavior. Thus, both bulk and nanocrystalline forms of GaN are n type and well above the metal-insulator transition (MIT), the nanocrystals most likely as a result of incorporation of shallow donor oxygen atoms during synthesis. Carriers in both forms of samples exhibit the near-ideal characteristics of a degenerate Fermi gas of noninteracting spins. The observation of NMR signals from both atom types, Ga and N, allows for the direct spatial correlation of the local electronic structure at the two sites in the lattice, specifically the $s$-orbital character of the electronic wave function of conduction band electrons at the Fermi edge. The relative values of these carrier wave-function probabilities (nearly twice as great for the N atom as for the Ga) are in line with theoretical predictions. Analyses of $^{71}\mathrm{Ga}, ^{14}\mathrm{N}$, and $^{15}\mathrm{N}$ NMR results, including double-resonance 2D $^{15}\mathrm{N}{^{71}\mathrm{Ga}}$ measurements, reveal electronic disorder in the form of broad distributions of local metallic properties (Knight shifts) that are shown to be spatially correlated on a subnanometer scale.

The aim of this work is to study the adsorption of several hydrocarbons by MIL-101(Cr) in order to find trends in adsorption behavior and relate those to the physicochemical properties of the sorbates. The separation of saturated, unsaturated and cyclic hydrocarbons was investigated on MIL-101(Cr) by means of gas chromatography equipped with a filled column of the MOF. MIL-101(Cr), proved to be very efficient to separate mixtures of saturated, unsaturated or aromatic hydrocarbons. The influence of hydrocarbons branching is evaluated towards their separation. We found that linear hydrocarbons are eluted firstly in the case of the n -hexane isomers series. We also demonstrated that xylenes are partially separated by MIL-101: metaxylene and paraxylene being eluted together, before orthoxylene. MIL-101 behaves therefore as a shape selective material with respect to the isomers of n -hexane, or xylenes. This separation could be predicted by the Henry’s constants which are calculated from the adsorption isotherms of single components by MIL-101(Cr). It is then demonstrated that these Henry’s constants establish a new tool for predicting the separation efficiency of any sorbent.

Surfactant-templated layered silicates are shown to possess complex compositional, structural, and dynamic features that manifest rich and interrelated order and disorder at molecular length scales. Temperature-dependent 1D and 2D solid-state Si-29 NMR measurements reveal a chemical-exchange process involving the surfactant headgroups that is concomitant with reversible broadening of Si-29 NMR line shapes under magic-angle-spinning (MAS) conditions at temperatures in the range 205-330 K. Specifically, the temperature-dependent changes in the Si-29 transverse dephasing times T-2' can be quantitatively accounted for by 2-fold reorientational dynamics of the surfactant headgroups. Variable-temperature analyses demonstrate that the temperature-dependent Si-29 shifts, peak broadening, and 2D Si-29{Si-29} correlation NMR line shapes are directly related to the freezing of the surfactant headgroup dynamics, which results in local structural disorder within the silicate framework.

3 pages; International audience; The distribution of hexamethylbenzene (HMB) molecules among the cavities of a Na-Y zeolite has been investigated by both xenon-129 and multiple-quantum NMR. For samples prepared at 523 K, xenon NMR shows that HMB molecules are dispersed nonuniformly, entering and remaining in the first available zeolite cavities. After further heat treatment at 573 K, the xenon spectrum collapses to one single line characteristic of a macroscopically uniform molecular distribution; multiple-quantum NMR “counting” indicates spin clusters of about 16 hydrogens, consistent with one HMB (18 hydrogens) per cavity.

3 pages; International audience; The performance of zeolite-supported metal catalysts is known to depend upon preparatory treatments.' Homogeneous and reproducible metal dispersion can be achieved only through careful control of calcination and reduction conditions, since migration and agglomeration of cluster precursors can occur with subsequent loss of catalytic activitya2s3 In spite of considerable the transformations undergone by metal guest species during calcination and reduction have not been established, primarily due to difficulties in characterizing chemical intermediates which exist as cluster precursors. By using the lz9Xe NMR spectroscopy technique pioneered by Fraissard and c o -w~ r k e r s 'a~n'd~ a pplied to recent advantage by us and others,I4-l6 we gain insight into the detailed chemistry of metal-zeolite catalyst preparation. We report for the first time changes in the chemical environment of Nay-supported platinum guest species by monitoring shifts in the '29Xe resonance signal induced by different chemical and thermal treatments.

8 pages; International audience; Local and macroscopic distributions of adsorbed benzene, 1,3,5-trimethylbenzene, and hexamethylbenzene (HMB) molecules among the intracrystalline cavities of NaY zeolite samples have been investigated by Iz9Xe and multiple-quantum NMR. Xenon- 129 NMR, a sensitive probe of macroscopic distributions of molecules in zeolites at room temperature, demonstrates the importance of sample heat treatment in distributing an adsorbate homogeneously throughout a collection of zeolite particles. NaY samples containing organic guests adsorbed at temperatures near the bulk species' melting point produce multiple or broad xenon lines characteristic of macroscopic (Le,, interparticle) adsorbate concentration gradients. After thorough heat treatment of the samples at elevated temperatures, the xenon spectrum collapses to a single sharp line, consistent with a macroscopically uniform distribution. For HMB adsorbed in Nay, “counting” hydrogen clusters by multiple-quantum NMR reveals intraparticle HMB distributions consistent with one molecule per cavity at low loadings (average bulk loading I 1 molecule per cavity) and two molecules per cavity at higher loadings.